Four
From Wikinfo
Four (4) is the natural number following three and preceding five. In the NATO phonetic alphabet, the digit 4 is called "fower".
| 4 | |
|---|---|
| Cardinal | Four |
| Ordinal | Fourth |
| Numeral system | quartal |
| Factorization | <math>4 = 2^2</math> |
| Roman numeral | IV |
| Binary | 100 |
| Hexadecimal | 4 |
Mathematics
Four is the smallest composite number, its proper divisors being 1 and 2. Four is also a highly composite number. The next highly composite number is six. Four is the second square number.
In addition, <math> 2 + 2 = 2 \times 2 = 2^2 = 4 </math>. Continuing the pattern in Knuth's up-arrow notation, <math> 2 \uparrow\uparrow 2 = 2 \uparrow\uparrow\uparrow 2 = 4</math>, and so on, for any number of up arrows.
A four-sided plane figure is a quadrilateral, sometimes also called a tetragon. A solid figure with four faces is a tetrahedron. The regular tetrahedron is the simplest Platonic solid.
The smallest non-cyclic group has four elements; it is the Klein four-group. Four is also the order of the smallest non-trivial groups that are not simple.
A flat two-dimensional map can always be colored using only four different colors so that all adjacent regions are colored with a different color; three colors are not sufficient. This is the four color theorem.
Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four square numbers. Three are not always sufficient; 7 for instance cannot be written as the sum of three squares.
Other fields
Four (四 pinyin si4) in considered an unlucky number in Chinese culture because it sounds like the word "death" (死 pinyin si3).
The four Greek classical elements are fire, air, water, and earth.
Usages include:
- Tetramorph
- Gang of Four
- Four Daughters
- Four noes and one without
- Four Modernizations
- Four Non Blondes
- Four great ancient capitals of China
- Four corner method
- Four-character idiom
- Four cardinal principles
- Four Books
Four is:
- The atomic number of beryllium.
- the figurative number of corners that the world has.
- the number of musicians in a quartet.
- The number of strings on a violin, a viola, a cello and a double bass.
- In the expression "fourth estate", referring to journalists.
- The designation of Interstate 4, a freeway in Florida.
- an album by the Blues Traveler: Four (album)
- The year AD 4.
References
- Adapted from the Wikipedia article, "Four" http://en.wikipedia.org/wiki/Four, used under the GNU Free Documentation License
